A 10-year study of humpback dolphins finds that romance is flourishing in the waters off Australia.
While humans may court one another with gifts of flowers or maybe jewelry, male dolphins aren’t that different; just swap red roses for sea sponges. This is according to scientists who studied Australian humpback dolphins (Sousa sahulensis) between the years 2008 and 2017 along a stretch of tropical Western Australian coastline.
Not only did the researchers document numerous occasions of male dolphins presenting – and sometimes tossing – sea sponge specimens to females, but they also often performed a show of strength. Much like a strongman pose, the so-called “banana pose” is a distinct physical posturing in which the animal appears to flex, with the rostrum, head and, sometimes tail rising above the water surface. Oh, and sometimes there was trumpeting from the blowhole as well – because when gifts and machisimo don’t work, why not make some noise?
The use of objects in sexual displays by non-human mammals is rare, write the researchers – but they reject the idea that the behavior could have other purposes, such as entertainment or food, writing:
We report on multi-modal sexual displays involving object presentation by males in a non-human mammal. Some male Sousa present marine sponges and engage in physical posturing and acoustic displays. Our data suggest that marine sponge presenting in Sousa is part of a sexual display rather than, for example, a form of object play or foraging.
Photo: (a) Adult male Sousa presenting a large marine sponge in proximity to adult females (F. Smith); (b) Adult male Sousa tossing a sponge toward an adult female (A. Brown); (c) Adult male Sousa (right of frame) performing “banana pose” in proximity to adult female (A. Brown); (d) Adult male Sousa physically posturing and emitting a trumpeting sound (for ca. 30 seconds) while swimming immediately behind an adult female (S. Allen).
Interestingly, the choice of marine sponge is significant as it speaks to a particular male’s vigor, agility and intelligence. Large marine sponges are not easily pried from their substrate and often contain chemical defenses.
Sponges may therefore require dexterity and strength to remove, while conceivably exposing the dolphin to both discomfort from chemical defences and greater risk of shark attack while otherwise engaged. Obtaining and presenting the sponge may also represent a signal of cognitive ability, thereby indirectly indicating male quality where higher cognitive performance is linked to male mating success.
Remarkably, the study also documents how male dolphins work together in pairs in order for one of them to mate with a female. It is unusual to see such alliances in a sexual context, note the authors, because conception cannot be shared. In humans, we might call number two the wingman – but in non-human animals it is rare. Which is all to say, dolphins keep revealing more behavior that we humans can relate to.
“Taken together, these findings suggest a hitherto unrecognized level of social complexity in Australian Sousa. Despite their vastly different evolutionary histories, some cetacean species appear to have converged on similar complexity and flexibility in behaviour and social systems as some of the more cognitively advanced bird and great ape species,” the authors conclude, “including our own.”
You can read the whole study here: Multi-modal sexual displays in Australian humpback dolphins